This invention relates to a method and apparatus for processing a video signal.
FIG. 1 illustrates in block form a typical arrangement of video production equipment. In production of a television news program, a camera 2 provides a full-field video signal VCAM representing a picture of a news reader 4 (FIG. 2A) against a background 6. A chroma keyer 8 generates a chroma key signal KCRM that defines an area of picture that has a particular color, such as a saturated blue. Signal KCRM is high for points that are the selected color and is low for points that are not the selected color. Typically, the only portion of the picture that is the selected color is a panel 12 of background 6.
A character generator 10 provides a full-field video signal VCHR representing a picture of a caption containing the title of the program (FIG. 2B), and an accompanying key signal KCHR that defines the boundaries of the characters. The key signal is high for points within the outlines of the characters shown in FIG. 2B and is low for points outside those outlines. Video signal VCHR and key signal KCHR are selected by a switcher 14 and applied to a digital video effects device 18, which spatially transforms the picture (compresses and translates it). For example, the operator might cause device 18 to compress the picture to the size of panel 12 and position it in the same location of the raster as panel 12. Device 18 processes key signal KCHR in essentially the same fashion as it processes video signal VCHR. The fill video signal VFIL provided by device 18 is combined with the camera signal VCAM in a multiplier 22 that operates under control of the compressed and translated key signal KFIL. In this fashion, a video signal VOUT representing the picture shown in FIG. 2C is generated at the output of the multiplier.
It is conventional to cut from the picture of the news reader to a target picture having some relevance to the events to which the news reader is referring, for example a picture illustrating launch of a spacecraft as shown in FIG. 2D. In order to provide a smooth transition from one picture to the other, the target picture may be shown briefly in the panel area of the background in place of the caption (FIG. 2E). The target picture is represented by a video signal VVTR provided by a videotape recorder 26. Switcher 14 selects video signal VVTR and key signal KCRM. Device 18 compresses and translates the picture represented by signal VVTR to fit it in the area defined by key signal KCRM, and the picture represented by the video signal VVTR appears in the panel area of the background. Video effects device 18 then expands the combined picture and shifts it so that the picture represented by the video signal VVTR occupies the entire raster.
In order to accomplish the effects that have been described above, it is necessary that digital video effects device 18 have two digital processing channels. One channel processes the video signal and the other processes the key signal.
The Grass Valley Group DPM 100 Digital Video Effects Generator can be used to carry out spatial transformation of a picture represented by a video signal. The main functional blocks of the DPM 100 device include a luminance keyer 32, i.e., a keyer that generates a key signal in dependence on the level of the video signal received by the keyer, and a digital picture manipulator 34, arranged as shown in FIG. 3. The simplest form of the DPM 100 device has a siggle digital processing channel and is designed to receive a video signal but no key signal, the key signal to accompany the output video signal being provided by luminance keyer 32. Where a substantial difference in luminance levels exists between, for example, foreground and background portions of the picture, a luminance keyer may be entirely satisfactory. However, if luminance keyer 32 of the DPM 100 device received the signal VCHR provided by character generator 10, the key signal generated by the luminance keyer might not define clearly the boundaries of the characters because there might not be a significant difference between the luminance level of the characters and the luminance level of the background. Accordingly, the DPM 100 device cannot be used as the digital video effects device 18 in the arrangement shown in FIG. 1.